1. Field of the Invention
The present invention relates to a fire alarm system including a plurality of terminal devices and a fire control panel connected to the plurality of terminal devices via power/signal lines.
2. Description of the Related Art
There is a fire alarm system in which a plurality of terminal devices such as fire detectors are connected to a fire control panel via a pair of power/signal lines. In this fire alarm system, a pulse voltage (pulse signal) is supplied from the fire control panel to the terminal devices, and each of the terminal devices responds to the voltage supplied from the fire control panel. With unique addresses assigned to the plurality of terminal devices, the fire control panel calls the terminal devices in order based on the addresses. Then, the fire control panel receives information returned from the called terminal devices to collect information such as determination of fire and statuses of the terminal devices (see, for example, Japanese Patent Application Laid-open No. 2002-288751).
The terminal device includes a constant voltage circuit, and returns a signal to the fire control panel in combination with the pulse voltage (referred to as “L voltage” when the voltage is lowered) for lowering a voltage of the pair of power/signal lines to a predetermined voltage by turning on/off the constant voltage circuit. At this time, a transmission current is caused to flow through the power/signal lines, and a line voltage of (transmission current)×(line resistance) occurs due to the transmission current and a line resistance of the power/signal lines.
The pulse voltage returned by the terminal device becomes (predetermined voltage)+(line voltage), and is received by the fire alarm system. This leads to a problem in that the fire alarm system fails to discriminate the pulse voltage returned by the terminal device when the line voltage becomes higher. In order to prevent this, the line resistance is previously defined based on a maximum transmission distance of the power/signal lines that can be extended from the fire alarm system. Then, a value of a current supplied to the power/signal lines and an upper limit value of the line voltage are limited based on the maximum transmission distance (maximum line resistance).
However, when a current limiting value with respect to the power/signal lines is uniquely limited, the maximum line resistance of the power/signal lines is uniquely limited in turn. On the other hand, a length of the power/signal lines, the number of connected terminal devices, and the like vary depending on an environment in which the fire alarm system is installed, and it is desired to provide a system that can flexibly handle the situations in accordance with an installation location or the like.